Field of the Invention
The present invention relates to a technology for diagnosing deterioration of an exhaust gas purification apparatus provided in an exhaust passage of an internal combustion engine and more particularly to a technology for diagnosing deterioration of an exhaust gas purification apparatus having a selective catalytic reduction catalyst (SCR catalyst).
Description of the Related Art
In a known method of diagnosing deterioration of an exhaust gas purification apparatus provided in an exhaust passage of an internal combustion engine, deterioration of the exhaust gas purification apparatus is diagnosed by using measurement values of air-fuel ratio sensors or oxygen concentration sensors provided respectively in the exhaust passage upstream of the exhaust gas purification apparatus and in the exhaust passage downstream of the exhaust gas purification apparatus. Specifically, the quantity of oxygen (O2) the exhaust gas purification apparatus can store (which will be hereinafter referred to as the “oxygen storage capacity”) is calculated from the difference between the outputs from the aforementioned sensors arranged at two locations at the time when the air-fuel ratio of the exhaust gas flowing into the exhaust gas purification apparatus is changed from a lean air-fuel ratio higher than the theoretical air-fuel ratio to a rich air-fuel ratio lower than the theoretical air-fuel ratio, and deterioration of the exhaust gas purification apparatus is diagnosed on the basis of the oxygen storage capacity (see for example, PTL 1). The aforementioned difference between the outputs will be hereinafter referred to as the “sensor output difference”.
PTL 2 discloses an exhaust emission control device of the internal combustion engine has the TWC arranged on the upstream side of an exhaust passage, an LNC arranged on this downstream side, an estimating means of a NOx quantity adsorbed to the TWC, and an estimating means of a reducing agent quantity consumed by the TWC, and has a correcting means for correcting the reducing agent quantity supplied to the LNC by taking into consideration NOx adsorbing capacity of the TWC and/or an increase-decrease in reducing agent consumption, a first learning correcting means on a change in the reducing agent consumption caused by the deterioration in the TWC, and a second learning correcting means on a change in a NOx adsorbing quantity and/or NOx reducing performance caused by the deterioration in the TWC.
PTL 3 discloses the diagnosis apparatus calculates, after activation of the catalyst or others, a difference (Vin−Vout=ΔV) between an output value Vin of the inlet air-fuel ratio sensor and an output value Vout of the outlet air-fuel ratio sensor, and diagnoses the catalyst based on the ΔV.